Defective lung microvascular arborization is a hallmark of bronchopulmonary dysplasia (BPD), a lung disease that develops in premature infants. Although clinical studies identify sepsis as a major risk-factor for BPD the mechanisms by which sepsis contributes to pulmonary vascular remodeling remain poorly understood. Prior work from our laboratory indicate that bacterial ligands like lipopolysaccharide (LPS) released during sepsis cause endothelial immune activation; a switch from a quiescent to an inflammatory endothelial phenotype. The overarching goal of this application is to understand the mechanisms by which sepsis-induced pulmonary endothelial immune activation contributes to inflammation and altered angiogenesis in the developing lung. Ongoing studies suggest that NADPH oxidase 2 (NOX2), an oxidoreductase enzyme belonging to the NOX family regulates LPS-induced lung endothelial immune activation. NOX2-mediated endothelial activation promotes lung inflammation by inducing intercellular adhesion molecule-1 (ICAM-1) and cytokine expression. Further, NOX2 also regulates sepsis-induced lung Angiopoietin-2 (ANGPT2) expression, which stimulates angiogenesis. Interestingly, in our mouse model of sepsis lung endothelial activation and inflammation are suppressed in NOX2-/- mice. These pilot data support a key role for endothelial NOX2 in regulating sepsis- induced lung pro-inflammatory angiogenic signaling, which is examined in Aim 1. LPS-mediated ANGPT2 signaling mediates autocrine angiogenesis through its receptor TIE2 in human lung endothelial cells. Further, ANGPT2 signaling through TIE2 induces expression of tip cell endothelial specification markers such as Delta- Like 4 (DLL4) preferentially over stalk cell specification markers like Jagged-1 (JAG1) in vitro. In neonatal mice sepsis robustly induces lung expression of DLL4 and KDR (VEGF receptor). Further, sepsis also programs aberrant angiogenesis in the developing retina. These data provide new insights into the mechanisms by which ANGPT2 regulates sepsis-induced angiogenesis in the neonatal lung; the focus of our studies in Aim 2. Given these preliminary results, we hypothesize that endothelial NOX2 regulates sepsis-induced lung inflammation and ANGPT2 expression, which directs altered angiogenesis in the neonatal lung by modulating the DLL4-Notch tip vs. stalk cell development . This hypothesis will be addressed in the experiments of the following Specific Aims: 1) Investigate whether endothelial NOX2 regulates lung endothelial immune activation and inflammation in sepsis; and 2) Determine the role of sepsis-induced ANGPT2 on DLL4-Notch directed lung angiogenesis. We expect that proposed studies will identify endothelial NOX2 and ANGPT2 as key molecular players regulating sepsis-induced inflammation and altered angiogenesis in the neonatal lung. Significant gains in our understanding of endothelial NOX2 signaling, ANGPT2-directed angiogenesis are also anticipated, which will inform future therapeutic strategies to decrease BPD burden in premature infants.